Seen on an Imperial-class Star Destroyer are ion drives.
Most starships depended on a particular type of engine, known as the ion engine or ion drive, to achieve sublight speeds. Power cells, liquid chemical reactants, onboard generators, or essentially any mechanism capable of delivering adequate power could fuel these ion engines. Through internal fusion reactions, the engine generated a stream of intensely charged particles, which were then propelled through the engine's exhaust at velocities approaching the speed of light. The term ion rockets was also sometimes used to refer to ion engines.
Because ion engines produced slightly radioactive waste products, technicians working around them were required to wear safety equipment. Due to this radioactivity, as well as the disruptive effect of ions on electronics and electrical systems, standard operating procedure dictated that most ships employ repulsorlifts for movement within an atmosphere, reserving the more potent ion drives for space travel.
The TIE fighters are perhaps the most well-known example of vessels using ion drives; their name is derived from the fact that they use Twin Ion Engines.
Ion drives are becoming a reality, as evidenced by the successful testing of the Dawn spacecraft's ion engines, which utilized Xenon as a propellant. It was active for a total of 27 hours while only using 10 ounces of propellant. The appearance of real ion thrusters is strikingly similar to that of the engines found on Star Destroyers. However, their thrust-to-weight ratio is quite low; this issue is resolved by the development of plasma engines, which are very similar but have a thrust-to-weight ratio that can be adjusted and scaled.